When cell divide they must accurately divide their chromosomes between the two daughters. This is accomplished by mitosis. Scientists have been studying chromosome movement during mitosis for over 100 years, and progress in the last 10 leads me to hope that the mechanism will be solved soon. During mitosis an elaborate structure of microtubules, termed the mitotic spindle, forms in the cytoplasm. Chromosomes then attach to microtubules of the mitotic spindle through their kinetochores. Chromosomes move by a mechanism that involves the elongation and shortening of microtubules at their kinetochores, combined with the action of motor-proteins. The latter are proteins that bind to microtubules and hydrolyse ATP to generate force. In this grant I propose experiments to tell us how the mitotic spindle forms, and how chromosomes move. This information is important both for our basic understanding of cells, and also for understanding and treating cancer, a disease in which cell division goes out of control. In order to understand spindle assembly we will purify proteins that help microtubules grow and shrink and determine how they work, and how they help in spindle assembly. We will assemble spindles in extracts from Xenopus (toad) eggs, and work out the role of specific proteins in this process. We are especially interested in a motor-protein that may move microtubules in the spindle and thus play an active role in the assembly process. In order to understand chromosome movement we will determine which motor proteins are present on the kinetochores of mammalian chromosomes by cloning them. We will then find out which direction they move, and how fast, which will tell us how they function in mitosis. We will also work with yeast kinetochores, which are easier to analyze biochemically. We will purify the motor-protein component of yeast kinetochores, and determine how it contributes to chromosome movement. Since motor-proteins must generate force, we will measure this force directly using a laser force-trap. This will tell us how much each motor contributes to the overall process of mitosis in living cells.